Sometimes
it is important to have explicit control
over the representation of a structure. The :type
option allows one to specify that a structure must be implemented
in a particular way, using a list or a specific kind of vector,
and to specify the exact allocation of structure slots to
components of the representation.
A structure may also be ``unnamed'' or ``named,'' according to whether
the structure name is stored in (and thus recoverable from) the structure.